I have this same issue on my brand new 2017, I really noticed it after i put on my new wheels. Didnt check my bed yet, but the strange thing is that when i jack up the truck by the axle, the tires were perfect, both at exactly 1". When the truck is on the ground, the rear passenger tire is 1" and the rear driver is 3/8". Do i just need time to settle? Any other thoughts. I plan on checking the bed as soon as I have the chance and it warms up.
